1. Field of the Invention
The present invention relates to a monitor, and more particularly to a dynamic focusing circuit for a monitor for compensating non-symmetry of a waveform of a dynamic focusing signal capable of occurring according to a frequency of a horizontal synchronization signal.
2. Prior Art
In general, a cathode ray tube (CRT) in a monitor converges thermal electrons generated from red (R), green (G), and blue (B) electron guns, and accelerates the converged thermal electrons. The converged and accelerated thermal electrons strike onto a fluorescent surface of the monitor through a shadow mask. The screen of the monitor is illuminated when the fluorescent surface is struck. Saw waveform signals flowing in horizontal and vertical deflection coils in the cathode ray tube deflect the converged and accelerated thermal electrons to display 2-dimensional image on the screen. A heater voltage of 6.3V is applied to a heater of the cathode ray tube to generate the thermal electrons from the heater, and a focus grid voltage and a screen grid voltage are required to converge the generated thermal electrons. Further, a high voltage of about 25 kV is applied to an anode of the cathode ray tube to accelerate the generated thermal electrons. Focus grids for focusing the thermal electrons generated from a cathode of the cathode ray tube have a static focus grid and a dynamic focus grid. A parabolic waveform signal, which is a dynamic focus signal, is applied to the dynamic focus grid for dynamic focusing.
FIG. 1 is a block diagram for showing a conventional dynamic focus circuit for a monitor. As shown in FIG. 1, the conventional dynamic focus circuit 100 comprises a synchronization process section 101, a waveform control signal generation section 102, a focus signal generation section 103, a buffer 104, a parabolic waveform signal output section 105, and a gain control section 106. The synchronization process section 101 includes Exclusive OR gates (not shown), and inputs a horizontal synchronization signal (H.sync) and a vertical synchronization signal (V.sync). The synchronization process section 101 outputs a horizontal and a vertical synchronization signal of one polarity regardless of polarities of the inputted horizontal and vertical synchronization signals. The waveform control signal generation section 102 includes a plurality of multivibrators (not shown) which generates a start signal and a stop signal according to inputs of a horizontal and a vertical synchronization signal. The focus signal generation section 103 generates horizontal and vertical focus signals of a parabolic waveform according to an input of the start signal of the waveform control signal generation section 102, and stops the generation of the horizontal and vertical focus signal according to an input of the stop signal of the waveform control signal generation section 102. The buffer 104 is constructed with general transistors, and generates a dynamic focus signal by summing horizontal and vertical focus signals generated from the focus signal generation section 103. The parabolic waveform signal output section 105 inputs and amplifies the dynamic focus signal to output a dynamic focus signal of a high voltage. The dynamic focus signal of a high voltage is applied to the dynamic focus grid (not shown) of the cathode ray tube through the flyback transformer 160.
Further, the primary winding of the flyback transformer 160 inputs a B+ voltage (scan voltage) from a direct current-direct current converter (DC-DC converter) 120 which is supplied with a direct current voltage from a switching mode power supply (SMPS) 100. The primary winding of the flyback transformer 160 is connected to the horizontal synchronization signal output section 140 and a horizontal deflection coil (H.DY) 150. The horizontal synchronization signal output section 140 is connected to a horizontal oscillation/drive section (130). The horizontal synchronization signal output section 140 supplies a saw waveform signal to the horizontal deflection coil 150 according to an output of the horizontal oscillation/drive section 130. The gain control section 106 inputs a mode control signal C31 from a microcomputer (not shown). The gain control section 106 controls the peak-to-peak voltage V.sub.pp of a horizontal dynamic focus signal of a parabolic waveform. As is well known, the thermal electrons generated in the cathode ray tube form an electron beam. The electron beam forms a focus on the screen of a monitor, and the diameter of the focus increases as the electron beam moves to the edge of the screen from the center of the screen. That is, since the diameter of a focus formed around the edge of the screen of the monitor gets larger than that of the focus formed on the center of the screen, clearness and sharpness of an image is degraded around the edge of the screen. In order to compensate for the degradation of the clearness and sharpness, a high resolution monitor uses a dynamic focus signal as a compensation waveform for an exact focus of the electron beam to be made on the screen of the cathode ray tube. The compensation waveform is controlled by a combination of the cathode ray tube and deflection coils. The peak-to-peak voltage V.sub.pp for the compensation tends to increase as the screen of a monitor gets larger. Accordingly, hundreds of voltage are required for the dynamic focusing signal, and such voltage is obtained from a flyback transformer 160.
FIG. 2A and FIG. 2B are views for showing a horizontal dynamic focus signal and a vertical dynamic focus signal, respectively. As shown in FIG. 2A, a dynamic focusing signal for compensating at the left and right edges of a screen is generated at every horizontal period (1II), and the dynamic focusing signal has a parabolic waveform signal which has a concave portion in the center portion thereof. As shown in FIG. 2B, a vertical dynamic focusing signal for compensating around top and bottom edges of the screen is generated at every vertical period (1V). The vertical dynamic focusing signal is a parabolic waveform signal which has a concave portion in the center portion thereof. The peak-to-peak voltage V.sub.pp around both edges of the parabolic waveform signal increases as the screen of the monitor gets larger in size, as described above. In general, the peak-to-peak voltage V.sub.pp ranges from 300V to 400V in case of the horizontal parabolic waveform signal, and ranges from 100V to 150V in case of the vertical dynamic focus signal. Such horizontal and vertical dynamic focus signals are generated from the focus signal generation section 103 in synchronization with horizontal and vertical synchronization signals. The generated horizontal and vertical dynamic focus signals are applied to the flyback transformer 160 through the buffer 104 and the parabolic waveform signal output section 103.
FIG. 3A to FIG. 3F show waveforms for dynamic focus signals generated from a conventional dynamic focus circuit according to frequencies of horizontal synchronization signals, where the frequencies of horizontal synchronization signals are generated according to modes such as a 640.times.480 mode, 800.times.600 mode, etc. (or a VGA (Video Graphic Array) mode, a super VGA mode, etc.). Hereinafter, a frequency of the horizontal synchronization signal is called a "horizontal frequency" for simplicity. FIG. 3A is a waveform for a horizontal dynamic focus signal at a VGA mode (the horizontal frequency=31.5 kHz). As shown in FIG. 3A, the horizontal dynamic focus signal is non-symmetrical with respect to the vertical center line. Further, the horizontal dynamic focus signal is shifted to the left with respect to a video display area. The difference between a voltage at the start point A and the end point B of the horizontal dynamic focus signal is 167V.
FIG. 3B is a waveform for a horizontal dynamic focus signal at the horizontal frequency of 37 kHz. As shown in FIG. 3B, the horizontal dynamic focus signal is more symmetrical than that at the VGA mode with respect to the vertical center line, and shifted to the left with respect to the video display area. The difference between a voltage at the start point C and a voltage at the end point D of the horizontal dynamic focus signal is 167V.
FIG. 3C is a waveform for a horizontal dynamic focus signal at the horizontal frequency of 43 kHz. As shown in FIG. 3C, the dynamic focus signal is nearly symmetrical with respect to the vertical center line, and shifts to the right with respect to the video display area. The difference between a voltage at the start point E and a voltage at the end point F of the dynamic focus signal is 66V.
FIG. 3D is a waveform for a horizontal dynamic focus signal at the horizontal frequency of 46 kHz. As shown in FIG. 3D, the horizontal dynamic focus signal is nearly symmetrical, and shifted to the left with respect to the video display area. The difference between a voltage at the start point G and a voltage at the end point H is 109V.
FIG. 3E is a waveform for a horizontal dynamic focus signal at the horizontal frequency of 53 kHz. As shown in FIG. 3E, the horizontal dynamic focus signal is completely symmetrical with respect to the vertical center line, and not shifted with respect to the video display area. The difference between voltages of the start and end points I and J is 0V.
FIG. 3F is a waveform for a horizontal dynamic focus signal at the horizontal frequency of 64 kHz. As shown in FIG. 3F, the horizontal dynamic focus signal is nearly symmetrical with respect to the vertical center line, and slightly shifted to the right with respect to the video display area. The difference between voltages at the start and end points K and L is 130V.
As described above, in the conventional dynamic focusing circuit, the dynamic focus signal is nonsymmetrical with respect to the vertical center line thereof, and shifted in the video display area, so a drawback exists in that voltages for focusing around the left and right of the screen are different. Accordingly, in case of providing a voltage properly for focusing to the left edge of the screen, an image around the right edge of the screen becomes unclear and loses sharpness since a focusing voltage around the right edge is weakened. Conversely, in case of providing a voltage properly for focusing to the right edge of the screen, an image around the left edge of the screen gets unclear and unsharpened since a focusing voltage around the left edge is weakened. Further, an exact focusing compensation is hardly made since a voltage of a horizontal dynamic focus signal varies according to a horizontal frequency.